Three people have been taken to hospital with burns and a major incident has been declared around the site at Wood Treatment Ltd on Tunstall Road.

Specialist search teams are being used to locate the missing people and fire officers say there is the risk of further explosions.

Fire service teams said the scene was one of devastation when they arrived just after 9am - a four-storey building had collapsed.

They say 20 people were working at the mill site at the time. Crews treated the walking wounded as the site was evacuated.

Chief fire officer Paul Hancock said: "We are still dealing with four unaccounted people. We believe they are somewhere in the building. We will endeavour to deal with that issue in a few hours time once we get the incident under full control."

A 29-year-old woman suffered serious burns and blast injuries to her head, face, arms and chest, West Midlands Ambulance Service said.

She was flown to Queen Elizabeth Hospital, Birmingham, for further specialist treatment.

One person was flown by air ambulance to Wythenshawe Hospital in Manchester, with the other taken to Royal Stoke University Hospital, the North West Ambulance Service said.

A further 35 people have also been assessed at the scene by ambulance staff but were not injured.

People living nearby and a local school have been told to close their windows and doors because of smoke coming from the site, south of Macclesfield.

Witness Damian Malone told Sky News: "I heard this massive explosion, to find this big fireball rising from the building.

"The ground was actually shaking. It lasted for two, three or four minutes but as the ground seemed to settle there was another explosion on top of that and black smoke billowing out everywhere.

"The sound of the explosion was unreal, scary."

Local resident Charlotte Maher told Sky News: "We felt the house shake. We thought it was an earthquake. Then there was black smoke and we heard sirens.

"I got a call from my mum who works at Bosley (Primary) Schoool, and they'd had a visit from the police who told them there'd been a major incident and everyone should keep their windows and doors shut because of the smoke."

More than 40 pupils are at the school, which is about a mile from the explosion site

Patrick Jones works at Wood Treatment, and clocked off his shift at 6am.

He said:  "I am still worried about my mates. There are four of them that are still missing.

"I was on shift until six o'clock and the changeover for people coming on is at eight o'clock.

"I have tried to get down there to see if I can get but they won't let anybody through.

He said his employers were "brilliant" and added: "You couldn't ask for a better place to work.

"As long as my mates get out, that is all I am bothered about."